The read element in a magnetic disk system is a thin slice of material, located between two magnetic shields, whose electrical resistivity changes on exposure to a magnetic field. Magneto-resistance can be significantly increased by means of a structure known as a spin valve (SV). The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are two magnetic layers separated by a non-magnetic layer. The thickness of the non-magnetic layer is chosen so that the magnetic layers are sufficiently far apart for exchange effects to be negligible but are close enough to be within the mean free path of conduction electrons in the material. If the two magnetic layers are magnetized in opposite directions and a current is passed through them along the direction of magnetization, half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing the non magnetic layer. Once these electrons have crossed the non-magnetic layer, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one the layers must be permanently fixed, or pinned. The other layer, by contrast, is a “free layer” whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface of a magnetic disk). Structures in which the pinned layer is at the top are referred to as top spin valves. Similarly, in a bottom spin valve structure the pinned layer is at the bottom.
Shown in FIG. 1, is a schematic cross-section of a lead overlaid spin valve head. Seen there is GMR stack 11 that rests on insulating substrate 10 and is protected by capping layer 12. Although not directly connected to the GMR effect, an important feature of any spin valve structure is a pair of longitudinal bias stripes 13 that are permanently magnetized in a direction parallel to the long dimension of the device. Also seen in FIG. 1 are conductive leads 15 with tantalum underlayer 14. This design is considered to be one of the best candidates for narrow track width reading because of its high signal output and good stability. However, one big drawback is its track width broadening. This poor track width definition is due to the wide spreading current profile from lead to GMR stack, partly due to the high resistivity Ta underlayer 14 in the lead and partly due to the oxidation of Ta in the overlaid region during etching and annealing processes.
In a previously filed application, (Ser. No. 09/993,402 Nov. 6, 2001), it was described how a current channeling layer (CCL) 25 may be inserted between the lead underlay 14 and the GMR stack to minimize current spreading (see FIG. 2). The use of a CCL can effectively reduce the current spread caused by the Ta underlayer, but interface oxidation still remains a problem.
Another previously filed application (Ser. No. 09/931,155 Aug. 17, 2001) disclosed an approach wherein a canted soft adjacent ferromagnetic layer 33 (pinned by an antiferromagnetic layer 34) was used to stabilize the structure, as shown in FIG. 3. In this scheme, the magnetostatic field from soft adjacent ferromagnetic layer (SAL) 33, which is exchange coupled to antiferromagnetic film 34, is used to provide horizontal stabilization to the layer. The magnetization in the SAL is canted toward the transverse direction. The magnetostatic field generated by such a canted SAL layer biases the free layer magnetization in the center region along the horizontal direction while biasing the magnetization in the side region along the transverse direction. This is schematically illustrated in FIG. 6 where free layer 116, pinned layer 117, and seed layer 118 are seen.
The net effect of using a canted SAL is that the side region of the free layer has less flux sensitivity because of its transverse orientation. The requirement of interface cleaning is therefore significantly relaxed compared to the structure shown in FIG. 1. However, due to the high resistivity of AFM layer 13 and Ta underlayer 14, the current spreading is significant. During the actual manufacture of heads; the thickness and canting angle of the SALs may vary, due to processing variations, causing the bias field from the SALs to vary as well. In particular, if the bias field from the SAL is not large enough to pin the magnetization in the wing region, side reading will still occur.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,493,467, Cain et al. show a process for an MR with a canted pinning layer as do U.S. Pat. No. 4,967,298 (Mowry) and U.S. Pat. No. 6,188,495 (Wiitala). U.S. Pat. No. 5,637,235 (Kim et al.) and U.S. Pat. No. 6,292,335 B1 (Gill) are related patents.